Project Summary: Genetics-by-diet (GxD) interactions affect Ca absorption and influence the ability of adolescents to reach peak bone mass (PBM), a critical parameter for lifelong osteoporosis prevention. Low dietary Ca intake in children can limit PBM but physiological adaptation minimizes the consequence of low dietary Ca intake on bone by increasing vitamin D-regulated intestinal Ca absorption efficiency. Our long-term goal is to understand how genetics and diet interact to affect Ca metabolism and bone health. We used linkage mapping to identify novel genetic loci controlling of Ca and bone metabolism in growing BXD recombinant inbred mouse lines. We propose new studies to identify the functional polymorphisms affecting intestinal Ca absorption. Afterwards we will validate the contribution of several candidate genes in mouse models and translate these findings to humans. To meet our goals we have developed three specific aims: Aim 1: Identify expression level QTL (eQTL) in duodenum that underly the loci controlling Ca absorption in BXD RI mice. Two approaches will be used to identify non-coding polymorphisms that affect gene transcription and mRNA stability. These data will be integrated with ATAC-seq data and coupled to bioinformatics tools to identify regulatory site polymorphisms affecting Ca absorption. Aim 2: Determine the role of specific genes identified by genetic mapping to the control of basal Ca absorption and its adaptation to low dietary Ca intake. We hypothesize that polymorphisms affecting gene expression will modify basal Ca absorption. We have prioritized five genes identified within loci from our genetic mapping study to test this hypothesis. Deleting both gene alleles will test for an essential role of the gene in Ca absorption while deleting one allele will model the partial loss of expression that occurs when polymorphisms affect transcription factor binding sites. Finally, we hypothesize that the impact of disrupting these genes will be accentuated when low Ca diets increase intestinal Ca absorption efficiency. Aim 3: Determine the impact of candidate gene polymorphisms on Ca absorption in adolescent boys and girls. We will identify and test functional polymorphisms in the human orthologs of mouse genes identified in our mapping studies using Ca absorption and genotype data from 580 adolescent subjects. The expertise of our research team, our unique preliminary data, and our novel sample inventory enables us to understand the genetic controls of a critical physiologic determinant of PBM - Ca absorption ? and its major regulator ? Ca intake. We have identified novel genes controlling Ca absorption and we have the tools to validate them in mouse models and then translate this information to human adolescents. Upon completion, our findings have potential to define genetic factors that influence dietary Ca recommendations for achieving peak bone mass in humans.